In principle it's about how to build a custom HF SA. I'm sure you'll find better components for less money, or even use SDR, but good HF/microwave spectrum analysis until today relies on analog circuitry. It's amazing that sometimes you can't even find/buy a correct reactance, so you have to design something else yourself to make your VCOs work as intended.

The principles never change, but the implementation does. How to make good reliable HF filters with least effort? There it is.